Any color selecting shadow mask may be thought of as an electrically conductive or electron opaque webbing which insures that a given electron beam impinges only on a given portion of a CRT (cathode ray tube) screen. Conventionally, shadow masks comprise a thin (typically 0.006 inch) apertured sheet of metal, and must generally assume a curvature similar to the curvature of the CRT screen with which the mask cooperates. For spherically curved screens, the mask is generally likewise spherically configured and must be capable of self-maintaining its sphericity. Spherically configured dot masks (i.e., shadow masks of the type used with dot triad tubes) offer relatively high structural strength and are adequately self-rigid to maintain their sphericity.
Slit masks (i.e., those color selection electrodes which do not have circular apertures but rather an array of vertically oriented slits separated horizontally by columns of electron opaque material) are not self-rigid, cannot self-maintain sphericity, and instead must be supported in a cylindrical or planar contour. Since a cylindrical or planar configuration in a thin sheet or wire grid format is not self-supporting, such slit masks must be stretched taut in order to assure stability and precision in the location of the slits relative to the associated phosphor screen. As a consequence, a very rigid, heavy and costly frame must be provided for tensing a conventional slit mask. Other problems such as vibration of the electrically conductive slats or wires which are used to define the slits are also present in this type of mask. Examples, of slit masks of the type described may be found in U.S. Pat. Nos. 3,363,129, 3,573,528 and 3,638,063.
The structural disadvantages of the slit mask have been overcome to some degree by the slot mask. Slot masks are similar to slit masks since slot mask apertures comprise columns of vertically oriented slits horizontally separated by columns of electron opaque material. However, the slits are interrupted vertically by horizontal strips of metal known as "tie-bars" which act as strengthening bridges across the slits. Thus each slit becomes a plurality of slots. Conventionally, the tie-bars are provided in sufficient number and with sufficient individual width to impart enough structural strength to the mask to allow it to self-maintain a spherical or other curved configuration. However, since the slot mask tie-bars intercept a portion of the phosphor-exciting electron beams, the structural strength requirements, which dictate that the tie-bars be sufficiently wide and numerous, impose a predetermined maximum bound on electron beam transmission and in turn on reproduced picture brightness. Moreover, the slot mask tie-bars if sufficiently wide, may interfere with the electron beam raster pattern to form moire' patterns which may seriously degrade the reproduced picture quality.
Attempts to increase self-rigidity in any type mask by increasing thickness of the mask material encounter problems arising in standard etching and stretch forming methods of manufacture and create a detrimental hole close-down at non-zero beam angles.